| | Total (N = 3954) |
|---|
| | N | % | Mean | SD |
|---|
| Sex |
| Boys | 2022 | 51.1 | | |
| Girls | 1932 | 48.9 | | |
| Age (months) | | | 62.7 | 3.2 |
| Gestational age (weeks) | | | | |
| < 28 | 26 | 0.7 | | |
| 28 < = < 37 | 397 | 10.0 | | |
| 37 < = < 42 | 3388 | 85.7 | | |
| 42 < = | 143 | 3.6 | | |
| Birth weight (g) |
| < 1000 | 12 | 0.3 | | |
| 1000 < = < 1500 | 16 | 0.4 | | |
| 1500 < = < 2500 | 349 | 8.8 | | |
| 2500 < = < 4000 | 3531 | 89.3 | | |
| 4000 < = | 46 | 1.2 | | |
| Family structure |
| living with parents | 3532 | 89.3 | | |
| living with a single parent | 376 | 9.5 | | |
| Others | 46 | 1.2 | | |
| History of developmental abnormalities | 182 | 4.6 | | |
| Familial annual income |
| < JPY 2 million | 312 | 7.9 | | |
| JPY 2–4 million | 1242 | 31.4 | | |
| JPY 4–7 million | 1453 | 36.7 | | |
| JPY 7–10 million | 434 | 11.0 | | |
| JPY 10 million < = | 201 | 5.1 | | |
